Section 3.3 Study Guide
Vocabulary for Section 3.3
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Cell membrane
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Phospholipid
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Fluid Mosaic Model
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Selective permeability
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Receptor
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Ligand
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Hydrophobic
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Hydrophilic
Review Questions
1. What molecules make up a cell membrane? A cell membrane is made up of how many layers?
The cell membrane is made up of phospholipids, cholesterol, and proteins. Two layers.
2. What are the two parts to a phospholipid and why do they form a double layer?
The two main parts of the phospholipid are the fatty acid (lipid) tail and the polar (magnetically
charged) phosphate head. The polar, phosphate, head interacts with the water inside and outside the
cell (in the cell’s cytoplasm and the intercellular space), forming transient (short-lived) hydrogen
bonds. The nonpolar lipid tails are repelled by water and interact with each other inside the cell
membrane (pointing toward one another).
3. What part of a phospholipid is hydrophobic and what part is hydrophilic?
The polar phosphate head is hydrophilic (water loving) and the lipid (fatty acid) tail is hydrophobic
(water hating).
4. What is the fluid mosaic model of the cell membrane? How does the cell membrane behave like a
fluid?
The fluid mosaic model states that, first, the cell membrane is flexible, not rigid (it “self-heals,” as we
saw from our lab). The phospholipids in each layer move from side-to-side and can slide past one
another – behaving like a fluid. The mosaic part of the name refers to the transport proteins and
other molecules that are found in the membrane (they provide the “mosaic” part of the fluid mosaic
model’s name).
5. What type of membrane allows some but not all molecules to cross?
A selectively permeable (semipermeable) membrane.
6. What is selective permeability and how does it help to explain the behavior of our “naked egg?”
Selective permeability means that the membrane allows some, but not all, materials to cross. With
respect to our “naked egg,” we saw how the eggs membrane was permeable to water – thus
explaining why it expanded and shriveled depending on the changing concentration gradient.
7. What is a receptor, what do they do, and what types (we discussed two types) of receptors are
found in cells?
A receptor is a protein that detects a signal molecule and performs an action in response. The
receptor recognizes and binds to only certain molecules – ensuring the right cell gets the right signal at
the right time. There are two types of receptors: intracellular (inside of the cell) receptors and
membrane receptors.
8. Explain how cell membrane receptors transmit messages across the cell membrane? Sketch how
they do this (see diagrams in notes).
Membrane receptors bind to a signal molecule on the outside of a cell. Upon binding, the membrane
receptor changes shape, sending a message inside the cell.
9. Draw a cell membrane and label its parts (define what each part does).